1. Field of the Invention
This invention relates to new multistrata, photohardenable elements which in one embodiment can be used for preparing image-containing colored masks that are useful for copy preparation and related purposes. Such elements are useful with ultraviolet radiation for exposing and hardening other photohardenable elements.
2. Description of the Prior Art
In both the graphic arts and electronics industry, it has become common practice to use photohardenable elements therein because of their high uniformity, ease of preparation, lower cost and other well known factors. During the process of manufacture, copies must often be made which require the use of so-called "masks". These masks are copied from original "artwork" carefully prepared by draftsmen. One system, for example, prepares a copy of the artwork original on a silver halide lithographic type film. This system is commonly used during the preparation of printed circuits. The silver halide film must then be "bleached" to reduce the image density so that the operator may see through the image to line up the copy while working in photopolymer safelights. The safelight used here is that which masks out the ultraviolet portion of the spectrum toward which the photopolymer resist is sensitive and is usually a yellow color. Under this light, the black, developed silver prevents the operator from seeing through the image to line up the copy. The bleaching process, however, produces a brown color through which the operator may readily see while preparing subsequent copies. This visibility is very important as often the operator must place several image copies, one on top of the other, and the resultant "registry" must be perfect in preparing the subsequent copy. The inherent drawbacks of this system are obvious. There are numerous steps in producing the mask, such as exposure, development, bleaching, washing, etc., all done with silver halide safelights, which, because of the inherent nature of the photographic silver halide system, are different from the photopolymer safelight system. In addition, a defect known as "pin-holes" is common to the silver halide system and subsequent passage of ultraviolet light through these pin-holes produces unwanted exposure in the photopolymer copy. This defect is extremely undesirable in the printed circuit industry and cannot be tolerated. Hence, many masks must sometimes be made in order to produce one which is satisfactory, resulting in great waste and cost. In addition, the bleaching process removes excess silver which is either lost or which must be recovered from the processing solution which adds cost to the system. Finally, after several exposures, the bleached image reverts back to a highly blackened state, preventing the operator from obtaining registry, and the mask must be discarded. Thus, the life of this type mask is shortened by use.
Yet another system uses so-called "photomasks" for the preparation, for example, of integrated circuits by photolithographic techniques. These masks are copied by the process of photoreduction from the original artwork, as taught by Madlen et al. in "Integrated Circuit Engineering Basic Technology", Chapter 3, 4th Edition (1966), published by The Boston Technical Publishers, Inc. The original artwork, termed a "photo master", is prepared 200-1000 times the actual size desired and is zealously protected because of the high cost of preparation. Photomask copies of this original are usually made on, for example, optically clear photographic glass plates coated with a fine grain, high resolution silver halide emulsion, such as the "Lippman" type. Because of the delicate and fragile nature of these masks, only a limited number of copies can be made. In addition, they, too, suffer from pin-hole defects described above. Another type of mask commonly used for this process is taught by George and Seaman in "Photo Methods for Industry", October, 1967, pp. 66-67. These are the "metal-on-glass" type which are usually thin depositions of a Ni-Cr-Fe alloy or chromium on a glass substrate. A photohardenable resist layer is then coated on top and the original "photo master" imaged on this photoresist by known techniques well described in the above reference. The area of the photohardenable layer which is left unhardened is washed off by solvent development and the metal "etched" through to the glass in these areas with special etching solutions such as inorganic acids. The remainder of the hardened photoresist is then removed and the metal image cleaned thoroughly leaving a suitable photomask. The metal mask is far superior to the emulsion mask due to its durability and ability to reproduce the original with greater fidelity. However, their manufacturing cost is very high and the number of steps required to prepare the mask makes their use prohibitive in those areas where cost is a major factor. These metal masks also suffer from pin-hole defects and are also relatively opaque to visible light which makes it difficult to obtain good registry.